Particle transfer



Dec. 3, 1968 c. F. GALLO. JR 3,414,409

PARTICLE TRANSFER Filed April 30. 1965 CHARGE l EXPOSE FIG. 1

DEVELOP l TRANSFER FIG. 2

F IG. 3

I NVEN TOR.

A T TORNEYS CHARLES F. GALLO, JR.

United States Patent 0 3,414,409 PARTICLE TRANSFER Charles F. Gallo, Jr., Fairport, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 30, 1965, Ser. No. 452,290 11 Claims. (Cl. 961.4)

ABSTRACT OF THE DISCLOSURE This invention relates to the transfer of electrically attractable particles from a photoconductive member to a contacting transfer member, and is particularly useful in Xerographic copying.

Although the present invention is described with reference to its use in xerographic copying, it is broadly applicable to the transfer of particles adhering to a photoconductive member under the influence of electrical charge.

The imaging member, or plate, used in a widely known commercial form of xerographic copying is of the reusable type. Ordinarily, the particulate image formed on a reusable plate is not permanently aflixed thereto, but is transferred to another support member such as paper. After the particulate image has been transfered, the imaging member is usually readied for recycling. Commercially available reusable imaging members are of the drum or fiat plate type; both types typically comprise a photoconductive layer of, for example, vitreous selenium, overlying a metallic substrate.

Briefly, the xerographic copying process includes a first step of uniformly electrostatically charging the plate in darkness. The sensitized plate is then exposed to an optical image to selectively discharge the plate and form a latent electrostatic image. The latent image is developed by the application of electrically attractable marking particles which are typically, although not necessarily, selected to adhere to the charged ortions of the plate. The developed powder image is then transferred to a permanent support such as paper by contacting the paper to the image bearing plate and applying a second electrostatic charge to bring about particle transfer. After separation of the paper from the plate, the image is ordinarily fixed by exposure to heat.

It has now been found that the application of a second electrostatic charge may 'be eliminated in transferring the powder image from the photoconductive member to a transfer member. Thus, it becomes practicable to transfer developed images fr=om a xerographic plate to a transfer member which is too thick for particle transfer by means of an applied electrostatic charge of reasonable magnitude, or which is otherwise inaccessible to an electrostatic charging device. For example, the present invention provides a practicable method for labeling shipping cartons, printing on walls, or the like.

In accordance with the present invention, image transfer is accomplished by contacting the particulate image bearing member with a sheet of paper or other suitable transfer member and illuminating the combination as hereinafter described. Upon separation of the members,

a powder image is found to reside on the transfer memher which may then be fixed in the ordinary manner. It is noted that mere contact of the transfer member to the image bearing member, but without a suitable illumination step, will not produce satisfactory image transfer. This is apparently due to the tenacity of the electrostatic forces holding the powder image on the xerographic imaging member.

Accordingly, transfer of powder particles adhering by electrical forces to a photoconductive insulating layer to a transfer member in contact therewith is an object of the present invention. Improved transfer of a developed powder image from a xerographic plate to a transfer member is also an object of the present invention. Another object of the present invention is the formation of xerographic images on members inaccessible to a corona discharge device. Other objects of the present invention include, among others: elimination of the need for externally applied electrical forces in transferring a developed image from a xerographic plate to a transfer sheet; a more economical xerographic copying process; and, improved xerographic copying.

The present invention will be described in detail in connection with the accompanying drawings in which:

FIG. 1 is a flow chart of xerographic process steps;

FIG. 2 illustrates particle transfer in accordance with the present invention; and,

FIG. 3 represents the transferred particle image on a transfer sheet.

As shown in FIG. 1, reusable plate xerography ordinarily involves five separate process steps. Although certain steps may be combined, and the process otherwise varied, the five steps are typically carried out in the order indicated, and will serve to illustrate the present invention.

First, a substantially uniform electrostatic charge is applied to the imaging member, ordinarily referred to as the xerographic plate, which comprises a photoconductive insulating layer overlying a relatively conductive substrate. By passing the plate adjacent to an electrode raised to corona discharge potential, a substantially uniform electrostatic charge is conveniently applied to the plate by ion deposition or charge exchange. Suitable charging devices and methods are disclosed, for example, in U.S. 2,777,957. Any suitable charging method may be employed, however.

The charged, or sensitized, xerographic plate is then exposed to an optical pattern corresponding to the image to be reproduced. This may be done by means of a camera, a slide projector, or other optical means known in xerography and photography. The xerographic plate becomes selectively discharged in areas exposed to actinic radiation but retains substantially all its charge in unexposed areas. Accordingly, a latent electrostatic charge pattern is formed corresponding to the optical pattern. Although the described steps are widely used commercially, various other methods for forming a developable charge pattern are known and may be used instead. For example, an image-wise charge may be applied directly to a xerographic plate by pulsing a closely spaced shaped electrode with a sufficiently high voltage.

The latent electrostatic image is then made visible, or developed, by the application of oppositely charged marking particles. For example, if the xerographic plate is given a substantially uniform positive charge, which is then selectively dissipated by exposure to an optical pattern, marking particles charged to a negative polarity would be used to develop the latent image by the deposition of particles in the unexposed areas of the plate. Highly suitable developing methods and materials are disclosed in U.S. 2,618,551, 2,618,552, and 2,638,416, and elsewhere in the patent literature.

The particulate image formed on the xerographic plate is then transferred to a permanent support member so that the plate may be prepared for reuse in the xerographic process. It is at this point in the xerographic process that the transfer method of the present invention may be suitably applied, although it should be understood that the present particle transfer method is not limited to the xerographic copying.

In FIG. 2 an image transfer member 11 is shown in contact with a particulate image 12 and the photoconductive layer 13 of xerographic plate 10. The particulate image 12 is intended to represent a xerographic image developed in accordance with the above-described process steps or their equivalent. Therefore, prior to the application of the present transfer method, particulate image 12 is understood to adhere selectively to xerograhic plate 10 by electrical forces.

The illustrated assembly of the transfer member in contact with the developed xerographic plate is exposed to actinic radiation for the photoconductive layer by means of, for example, incandescent lamp 16. As seen in FIG. 2, the illumination source is positioned on the opposite side of the assembly from that of the transfer member. Accordingly, substrate 14 of xerographic plate 10 must be translucent, and preferably transparent. Glass is a highly suitable material for substrate 14 except that it ordinarily is not sufliciently electrically conductive for use in xerography. For this reason, the glass used for plate 10 must be specially treated to impart the necessary electrical conductivity, or a very thin transparent conductive layer must be incorporated into the plate structure. For example, a thin conductive layer of tin oxide, or the like, may be used, as represented in FIG. 2 at reference numeral 17. A highly suitable glass for present purposes is marketed under the name Nesa (Pittsburgh Plate Glass Company). Other suitable materials include, for example, conductive plastics, nonconductive plastics incorporating a translucent or transparent conductive coating or the like.

In accordance with an alternate embodiment of the present invention, the xerographic plate may be exposed through properly selected transfer members rather than through the plate substrate, as illustrated in FIG. 2. This embodiment is adaptable for use with transfer members sufficiently translucent to actinic radiation for the photoconductive layer. Moreover, to prevent shielding by the particulate image and assure adequate exposure, the radiation should impinge at. a relatively small angle with respect to the plate surface, unless, of course, the particulate image comprises radiation transmitting material.

It has been found that illumination of the particulate image-bearing photoconductive layer may be by means of a very brief flash of sutliciently high intensity light, or by relatively longer exposure to comparatively low level illumination. The former has the advantage of high speed and is, therefore more suitable for continuous process machines. The present invention is not restricted thereto, but is seen to incorporate the use of low level illumination as well.

The amount and frequnecy of the radiation required by the present invention is determined by the particular photoconductive material to which the particles adhere by electrical forces. Actinic radiation suflicient to create electron-hole pairs in the photoconductive layer is adequate to cause particle transfer in accordance with the present invention. It is believed that the actinic radiation is absorbed by the photoconductive layer and creates electronhole pairs which diffuse and substantially neutralize the electrostatic charge pattern. The particles, therefore, all having an opposite charge, mutually repel each other and are thus forced away from the photocondutcive layer to the transfer member with which they are in contact. Although there is no intention to limit the present invention to any theoretical explanation of the transfer process, observed results have been entirely consistent with the aforementioned mechanism. Particle transfer has been found to be especially effective when a relatively soft (as compared with the surface of the photoconductive layer) transfer member is used. Ordinary paper has been found to be highly satisfactory material for this purpose.

FIG. 3 schematically illustrated a transfer image formed in accordance with the present invention. After the illumination process explained in connection with FIG. 2 is completed, image member 11 is separated from the photoconductive layer. Transfer image 12' may then be permanently fixed to image member 11 by any conventional method such as exposure to heat or solvent vapor.

To further illustrate the present invention with specific examples: A substantially uniform positive electrostatic charge of approximately 600700 volts was applied to a xerographic plate comprising a thin film of vitreous selenium overlying a Nesa glass substrate by passing the plate close to an electrode raised to corona discharge potential. The sensitized plate was exposed in a camera to an optical image, and developed by cascading a mixture of electrical attractable marking materials and grossly larger carrier beads across the photoconductive layer, as more fully set forth in US. 2,618,552. The developer materials were selected so that the toner particles were triboelectrically charged to a negative polarity with respect to the remaining charged area of the xerographic plate. A sheet of paper was then placed in contact with the particle bearing surface of the plate, and the photoconductive layer was exposed to a flash of light energy of 50 joules. The paper was then separated from the plate, and a highly satisfactory particle image was found to have been transferred thereto.

For comparison purposes, the same procedure was repeated without the illumination step. Particle transfer was found to be insuflicient to produce an acceptable image. The effectiveness of the present invention is further illustrated by an additional test in which a second sheet of paper was pressed against the aforementioned particle image-bearing plate. Light was then flashed through the glass substrate and the paper was separated from the plate. Sufficient particle transfer was found to have occurred, producing a satisfactory and readable image on the paper.

Limitation of the present invention to the specific materials, applications, and embodiments disclosed herein for purposes of description and explanation is not intended. Rather it is intended that the appended claims encompass the disclosure and all reasonable equivalents.

What is claimed is:

1. The method of forming a particulate image on a copy sheet comprising:

(a) forming a latent electrostatic image on a xerographic plate comprising a photoconductive layer overlying a conductive substrate;

(b) applying electrically attractable marking particles to said latent image to form a developed image on said photoconductive layer, said developed image comprising image areas corresponding to charged areas of said xerographic plate and having a plurality of similarly charged marking particles thereon and background areas corresponding to uncharged areas of said xerographic plate and having isolated marking particles thereon;

(c) positioning a substantially uncharged, nonadhesive copy sheet adjacent said developed image and said photoconductive layer;

(d) selectively transferring substantially only said image areas of said developed image by uniformly illuminating said photoconductive layer to actinic radiation whereby the electrostatic forces of attraction between said marking particles comprising said image areas and said photoconductive layer are lessened whereby said similarly charged marking particles comprising said image areas mutually repel each other toward said copy sheet while the isolated marking particles in said background areas substantially remain adhering to said photoconductive layer; and

(e) removing said copy sheet from its position adjacent said photoconductive layer.

2. The method of claim 1 further including the step of fixing said transferred image to said copy sheet.

3. The method of claim 1 wherein said copy sheet is positioned in contact with said developed image on said photoconductive layer.

4. The method of claim 3 wherein said copy sheet is sufficiently flexible so that intimate contact can be achieved thereby preventing distortion of the image areas during transfer.

5. The method of claim 1 wherein said latent electrostatic image of charged and uncharged areas is formed by substantially uniformly charging said xerographic plate and exposing said charged xerogr-aphic plate to a pattern of light and shadow.

6. The method of claim 1 wherein said conductive substrate is transparent and said photoconductive layer is illuminated therethrough during transfer.

7. The method of claim 1 wherein said photoconductive layer is illuminated through said copy sheet during transfer.

8. The method of claim 1 wherein said exposure of step (d) comprises flash illuminating said photoconductive layer to actinic radiation.

9. The method of claim 1 wherein said photoconductive layer comprises vitreous selenium.

10. The method of claim 1 wherein said copy sheet comprises a sheet of paper.

11. The method of forming a particulate image on a copy sheet comprising:

(a) forming a latent electrostatic image on a xerographic plate comprising a photoconductive layer overlying a conductive substrate by exposing said Xerographic plate to a pattern of light and shadow;

(b) applying electrically attractable marking particles to said latent image to form a developed image on said photoconductive layer, said developed image comprising image areas corresponding to non-exposed areas of said xerographic plate and having a plurality of similarly charged marking particles thereon and background areas corresponding to the exposed areas of said xerographic plate and having isolated marking particles thereon;

(c) positioning a substantially uncharged, nonadhesive copy sheet adjacent said developed image and said photoconductive layer;

(d) uniformly exposing said photoconductive layer to actinic radiation whereby the electrostatic forces of attraction between said marking particles comprising said developed image and said photoconductive layer are lessened whereby said similarly charged marking particles comprising said image areas mutually repel each other toward said copy sheet while said isolated marking particles in said background areas substantially remain adhering to said photoconductive layer; and

(e) removing said copy sheet from its position adjacent said photoconductive layer.

References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson 96-1 2,924,519 2/1960 Bertelsen 961.4 2,951,443 9/1960 Byrne 961.4 X 3,043,685 7/1962 Rosenthal 96l.4 3,071,645 1/1963 McNaney 961.4 X 3,284,224 11/1966 Lehmann 117-17.5 3,322,537 5/1967 Giaimo 96l.1

NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner. 

